The present invention is directed to a collar for a watt-hour meter, and more particularly to a meter collar which is configured for a use with an interface circuit that facilitates using an on-site energy source in lieu of or in addition to commercial power from an electric utility company. The interface circuit may isolate the utility company's power lines when the on-site source is used, or it may permit the on-site power source to be used in parallel with commercial power. The invention is also directed to an interface circuit itself, whether it is used in conjunction with a meter collar or is instead associated with other components of a customer's private electrical distribution system, such as a meter socket box or a circuit breaker box.
Some customers of commercial electrical utility companies would like the option of using power which they, the customers, generate or store locally, or on-site. The customer's on-site power source may comprise, for example, a generator which is powered by a gasoline or diesel engine or a combustion turbine, a solar cell array which charges storage batteries that then supply electricity to an inverter for conversion to alternating current, a fuel cell and an inverter, or simply back-up storage batteries which are kept charged using commercial power and which supply power through an inverter when necessary.
Among the problems that typically confront a customer who wants the option of using either his or her on-site power source or the utility company's power is that the modifications in the wiring of the customer's private electrical distribution system (at the customer's residence, for example, or at a small business establishment receiving two-phase service) to accommodate the on-site power source are relatively expensive. Another problem is that the customer's electrical distribution system should either be isolated from the utility company's power lines, or connected to the power lines in a carefully controlled manner, when the on-site power source is used. The isolation option not only prevents possible damage to the utility company's distribution system and to the loads of other customers, it also protects technicians who may be working on the utility company's power lines from electricity generated by the customer's on-site power source. Safety is a paramount concern for utility companies, which train their line technicians to make sure the lines they are working are on are electrically isolated from the utility company's generating facilities. It is not customary for line technicians to also isolate the segments they are working on from the customers, however, unless the technicians have been specifically trained to do so.
Despite this potential hazard, it may desirable to permit a customer to use his or her own on-site power source in parallel with the utility's power, so that both the on-site power and the utility's power can be consumed by the customer's loads. If the utility permits, parallel operation would also allow excess on-site power to be coupled to the utility's power lines for distribution to other customers.
FIG. 1 illustrates a typical prior art arrangement illustrating how a utility's distribution system may be connected to the private distribution system of a customer who receives two-phase service (such as a residential customer with 110-volts/220-volt service or a small business owner with 110-volt/220-volt service). A utility substation 20 receives power at a high voltage from a generating station (not illustrated) and distributes this power (at a stepped-down but nevertheless relatively high voltage and in three phrases) to a network which includes a step-down transformer 22. The primary winding of the transformer 22 receives one of the phases from the substation 20, and the secondary winding in center-tapped. The center tap, which is grounded, is connected to a neutral power line 24. A "leg 1" of the secondary winding is connected to a leg-1 power line 26 and a "leg 2" of the secondary winding is connected to a leg-2 power line 28. The potential difference between the leg-1 power line 26 and the neutral line 24 is typically 110 volts (average) and the potential difference between the leg-2 power line 28 and is also typically 110 volts (average). However, leg-1 power line 26 is 180.degree. out of phase with the leg-2 power line 28. Consequently, a load which is connected between the neutral line 24 and either of the leg-1 or leg-2 power lines 26 and 28 receives 110 volts while a load connected between the leg-1 and leg-2 power lines 26 and 28 receives 220 volts. The two-phase service that is illustrated in FIG. 1 can thus supply power to both 110 volt loads and 220 volt loads that are connected to a customer's private distribution system.
FIG. 1 also shows the front side of a meter socket box 30 and the back side of a watt-hour meter 32. The socket box 30 has a recessed socket 34 with utility-side contacts 36 and 38 and customer-side contacts 40 and 42. Each of the contacts includes a pair of electrically conductive arms (not numbered). The socket 34 also includes a neutral contact 44 that is connected by a neutral service line 46 to the neutral power line 24 and to a neutral line 48 of the customer's private distribution system. The arms of the contact 36 are connected via a leg-1 service line 50 to the leg-1 power line 26 and the arms of the contact 38 are connected via a leg-2 service line 52 to the leg-2 power line 28. The arms of the contact 40 are connected to a leg-1 line 54 of the customer's distribution system while the arms of the contact 42 are connected to leg-2 line 56 of the customer's distribution system
With continuing reference to FIG. 1, the back side of the meter 32 is provided with four contacts, 58, 60, 62, and 64. When the meter 32 is plugged into the socket 34 as indicated schematically by arrow 66, the contact 60 is wedged between the arms of the contact 36 to form a connection, the contact 58 is wedged between the arms of the contact 38 to form a connection, the contact 64 is wedged between the arms of the contact 40 to form a connection, and the contact 62 is wedged between the arms of the contact 42 to form a connection. Meter 32 is an electromechanical meter having a Farraday motor and a gear train (not illustrated) which turns dials (not illustrated) when the motor rotates. The meter includes a low resistance winding (not numbered) between the contacts 58 and 62 and another low resistance winding (also not numbered) between the contacts 60 and 64 The meter also includes a high resistance winding (not numbered) between the contacts 62 and 64. The net result is that, when the meter 32 is plugged into the socket 34, the leg-1 line 54 of the customer's distribution system is connected to leg-1 power line 26, the neutral line 48 of the customer's distribution system is connected to neutral power line 24, and the leg-2 line 56 of the customer's distribution system is connected to the leg-2 power line 28. The meter 32 records the watt-hours consumed by the loads connected to the customer's distribution system.